Code is quite explanatory, change $u with the location of the KML file, code will output GPX XML.

Alternatively, download the code here.

<?php
	//enter location of KML file here
	$u = "http://code.google.com/apis/kml/documentation/KML_Samples.kml";

	function utcdate() {
		return gmdate("Y-m-d\Th:i:s\Z");
	}

	$u_parts = pathinfo($u); //array of url parts
	$u_ext = strtoupper($u_parts['extension']);
	if ($u_ext== "KML") {

		$dom_kml = new DOMDocument();
		$dom_kml->load($u);

		$dom_gpx = new DOMDocument('1.0', 'UTF-8');
		$dom_gpx->formatOutput = true;

		//root node
		$gpx = $dom_gpx->createElement('gpx');
		$gpx = $dom_gpx->appendChild($gpx);

		$gpx_version = $dom_gpx->createAttribute('version');
		$gpx->appendChild($gpx_version);
		$gpx_version_text = $dom_gpx->createTextNode('1.0');
		$gpx_version->appendChild($gpx_version_text);

		$gpx_creator = $dom_gpx->createAttribute('creator');
		$gpx->appendChild($gpx_creator);
		$gpx_creator_text = $dom_gpx->createTextNode('http://thydzik.com');
		$gpx_creator->appendChild($gpx_creator_text);

		$gpx_xmlns_xsi = $dom_gpx->createAttribute('xmlns:xsi');
		$gpx->appendChild($gpx_xmlns_xsi);
		$gpx_xmlns_xsi_text = $dom_gpx->createTextNode('http://www.w3.org/2001/XMLSchema-instance');
		$gpx_xmlns_xsi->appendChild($gpx_xmlns_xsi_text);

		$gpx_xmlns = $dom_gpx->createAttribute('xmlns');
		$gpx->appendChild($gpx_xmlns);
		$gpx_xmlns_text = $dom_gpx->createTextNode('http://www.topografix.com/GPX/1/0');
		$gpx_xmlns->appendChild($gpx_xmlns_text);

		$gpx_xsi_schemaLocation = $dom_gpx->createAttribute('xsi:schemaLocation');
		$gpx->appendChild($gpx_xsi_schemaLocation);
		$gpx_xsi_schemaLocation_text = $dom_gpx->createTextNode('http://www.topografix.com/GPX/1/0 http://www.topografix.com/GPX/1/0/gpx.xsd');
		$gpx_xsi_schemaLocation->appendChild($gpx_xsi_schemaLocation_text);

		$gpx_url = $dom_gpx->createElement('url');
		$gpx_url = $gpx->appendChild($gpx_url);
		$gpx_url_text = $dom_gpx->createTextNode($u_parts['dirname']);
		$gpx_url->appendChild($gpx_url_text);

		$gpx_time = $dom_gpx->createElement('time');
		$gpx_time = $gpx->appendChild($gpx_time);
		$gpx_time_text = $dom_gpx->createTextNode(utcdate());
		$gpx_time->appendChild($gpx_time_text);

		// placemarks
		$names = array();
		foreach ($dom_kml->getElementsByTagName('Placemark') as $placemark) {
			//name
			foreach ($placemark->getElementsByTagName('name') as $name) {
				$name  = $name->nodeValue;
				//check if the key exists
				if (array_key_exists($name, $names)) {
					//increment the value
					++$names[$name];
					$name = $name." ({$names[$name]})";
				} else {
					$names[$name] = 0;
				}
			}
			//description
			foreach ($placemark->getElementsByTagName('description') as $description) {
				$description  = $description->nodeValue;
			}
			foreach ($placemark->getElementsByTagName('Point') as $point) {
				foreach ($point->getElementsByTagName('coordinates') as $coordinates) {
					//add the marker
					$coordinate = $coordinates->nodeValue;
					$coordinate = str_replace(" ", "", $coordinate);//trim white space
					$latlng = explode(",", $coordinate);

					if (($lat = $latlng[1]) && ($lng = $latlng[0])) {
						$gpx_wpt = $dom_gpx->createElement('wpt');
						$gpx_wpt = $gpx->appendChild($gpx_wpt);

						$gpx_wpt_lat = $dom_gpx->createAttribute('lat');
						$gpx_wpt->appendChild($gpx_wpt_lat);
						$gpx_wpt_lat_text = $dom_gpx->createTextNode($lat);
						$gpx_wpt_lat->appendChild($gpx_wpt_lat_text);

						$gpx_wpt_lon = $dom_gpx->createAttribute('lon');
						$gpx_wpt->appendChild($gpx_wpt_lon);
						$gpx_wpt_lon_text = $dom_gpx->createTextNode($lng);
						$gpx_wpt_lon->appendChild($gpx_wpt_lon_text);

						$gpx_time = $dom_gpx->createElement('time');
						$gpx_time = $gpx_wpt->appendChild($gpx_time);
						$gpx_time_text = $dom_gpx->createTextNode(utcdate());
						$gpx_time->appendChild($gpx_time_text);

						$gpx_name = $dom_gpx->createElement('name');
						$gpx_name = $gpx_wpt->appendChild($gpx_name);
						$gpx_name_text = $dom_gpx->createTextNode($name);
						$gpx_name->appendChild($gpx_name_text);

						$gpx_desc = $dom_gpx->createElement('desc');
						$gpx_desc = $gpx_wpt->appendChild($gpx_desc);
						$gpx_desc_text = $dom_gpx->createTextNode($description);
						$gpx_desc->appendChild($gpx_desc_text);

						//$gpx_url = $dom_gpx->createElement('url');
						//$gpx_url = $gpx_wpt->appendChild($gpx_url);
						//$gpx_url_text = $dom_gpx->createTextNode($ref);
						//$gpx_url->appendChild($gpx_url_text);

						$gpx_sym = $dom_gpx->createElement('sym');
						$gpx_sym = $gpx_wpt->appendChild($gpx_sym);
						$gpx_sym_text = $dom_gpx->createTextNode('Waypoint');
						$gpx_sym->appendChild($gpx_sym_text);
					}
				}
			}
			foreach ($placemark->getElementsByTagName('LineString') as $lineString) {
				foreach ($lineString->getElementsByTagName('coordinates') as $coordinates) {
					//add the new track
					$gpx_trk = $dom_gpx->createElement('trk');
					$gpx_trk = $gpx->appendChild($gpx_trk);

					$gpx_name = $dom_gpx->createElement('name');
					$gpx_name = $gpx_trk->appendChild($gpx_name);
					$gpx_name_text = $dom_gpx->createTextNode($name);
					$gpx_name->appendChild($gpx_name_text);

					$gpx_trkseg = $dom_gpx->createElement('trkseg');
					$gpx_trkseg = $gpx_trk->appendChild($gpx_trkseg);

					$coordinates = $coordinates->nodeValue;
					$coordinates = preg_split("/[\s\r\n]+/", $coordinates); //split the coords by new line
					foreach ($coordinates as $coordinate) {
						$latlng = explode(",", $coordinate);

						if (($lat = $latlng[1]) && ($lng = $latlng[0])) {
							$gpx_trkpt = $dom_gpx->createElement('trkpt');
							$gpx_trkpt = $gpx_trkseg->appendChild($gpx_trkpt);

							$gpx_trkpt_lat = $dom_gpx->createAttribute('lat');
							$gpx_trkpt->appendChild($gpx_trkpt_lat);
							$gpx_trkpt_lat_text = $dom_gpx->createTextNode($lat);
							$gpx_trkpt_lat->appendChild($gpx_trkpt_lat_text);

							$gpx_trkpt_lon = $dom_gpx->createAttribute('lon');
							$gpx_trkpt->appendChild($gpx_trkpt_lon);
							$gpx_trkpt_lon_text = $dom_gpx->createTextNode($lng);
							$gpx_trkpt_lon->appendChild($gpx_trkpt_lon_text);

							$gpx_time = $dom_gpx->createElement('time');
							$gpx_time = $gpx_trkpt->appendChild($gpx_time);
							$gpx_time_text = $dom_gpx->createTextNode(utcdate());
							$gpx_time->appendChild($gpx_time_text);
						}
					}
				}
			}
		}
		header("Content-Type: text/xml");
		echo $dom_gpx->saveXML();
	}
?>

Let me know if you find any issues.

The XML for the map below was made with RoboGEO, which synchronises GPS data with photographs using the timestamp. The XML map data was then displayed as a Google map using my WordPress plugin thydzikGoogleMap.

One of the features of thydzikGoogleMap is the ability to show XML Google maps, while maintaining the internal links to photographs.